Certainly would check ALL of your connections coming in to the monitor. What you didn't say is if ALL of the cameras do the same thing or if the side cameras are OK and it is just the rear camera that acts up.
The cables connected rom the cameras to the monitor are shielded and should not be getting any interference from the engine IF the system is properly grounded to the chassis. When checking the connectors, also make sure that the ground connection to the monitor is secure.
If just your rear camera is acting up, you may also want to check the connector for the rear camera. On the systems that I have had, the rear camera has a short pigtail that is connected to it and you have to pull it out of the rear cap to insure that it is plugged in to the long cable running to the front monitor.